It is conventional in seismic prospecting to employ an array of colinear acoustic receivers. For example, in one conventional seismic prospecting technique, a single seismic source (such as an explosive source in a shallow shot hole or a seismic vibrator) and a number of acoustic receivers are disposed substantially colinearly on the earth's surface. Alternatively, in the class of conventional techniques known as vertical seismic profiling (VSP), a receiver is positioned at a number of locations (depths) in a borehole, spaced along the borehole axis. A seismic source is fired and the signal is recorded at each geophone position. A VSP technique wherein a seismic source is positioned at (or near) the intersection of the borehole and the earth surface, is known as a "zero-offset VSP" technique. A VSP technique wherein a seismic source is positioned at (or near) the earth surface, but horizontally offset from the borehole, is known as an "offset VSP" technique.
Conventional seismic prospecting techniques may be employed to determine the position of a planar subterranean reflector. Throughout this application it will be assumed that the "strike" of each referenced planar reflector (the direction of the intersection of the reflector and a horizontal plane) is known, and that it is desired to determine the dip angle of the reflector, the depth (vertical distance) below the earth surface of the reflector at a particular point relative to a known position on the earth surface (such as the mouth of a borehole), and more importantly, the position of each reflection point that is represented on the seismic record. The term "imaging" the reflector will be used herein to denote determining the position of each point on the reflector that is represented on the seismic record.
In seismic prospecting, acoustic signals received at each of a number of acoustic receivers are typically recorded after these signals have propagated away from a common source location through a subterranean earth formation. Most commonly, a single seismic signal propagates away from the source as a result of a single event of operation ("shot") of the source, and each recorded acoustic signal possesses a portion of the wave energy resulting from the single shot. Alternatively, each recorded signal may be associated with a different shot, where each shot occurs at the same source location.
The recorded signals associated with a single source location but with different receiver locations will be denoted in this Specification collectively as a single shot gather of seismic traces. The receiver locations are assumed to be colinear or approximately so. In accordance with conventional techniques, the traces in the gather are displayed side-by-side as a "seismic section" (or "seismic record"). Reflection signals along the traces associated with the same subterranean reflector will fall along a generally hyperbolic curve known as a traveltime curve that may sometimes be identified by visual inspection of the seismic section.
Conventional methods for determining the actual subsurface position of a reflector from the gather of traces have required that the apex of the hyperbolic traveltime curve be identified. The present invention eliminates the need to identify the apex of any traveltime curve. Instead, the invention enables determination of the dip of a plane reflector, and the reflector's position along a line of receiver locations, from any two or more points on the traveltime curve. The inventor has recognized that these two parameters are the identifying characteristics of any seismisc reflector from which the position of each reflection point which is represented on the seismic record may be calculated with respect to a user-chosen coordinate system. The invention permits use of a reduced data set, resulting in considerable reduction in data collection and processing time and expense, relative to conventional techniques. Because data processing in accordance with the invention is so simple, it is anticipated that the invention will enable real-time processing and interpretation to be done in the field.